Operations and Significant Accounting Policies - Leases (Details) - USD ($)
$ in Millions |
12 Months Ended |
|
Dec. 31, 2019 |
Dec. 31, 2018 |
| Lessee, Lease, Description [Line Items] |
|
|
|
| Lessee, Operating Lease, Residual Value Guarantee, Description |
|
Our largest operating lease is for the dragline at BNI Energy which includes a termination payment at the end of the lease term if we do not exercise our purchase option.
|
|
| Operating Lease, Right-of-Use Asset |
[1] |
$ 28.6
|
$ 0.0
|
| Operating Lease, Cost |
|
9.4
|
|
| Operating Lease, Payments |
|
9.4
|
|
| Operating Lease, Liability, Current |
[2] |
6.9
|
0.0
|
| Operating Lease, Liability, Noncurrent |
[3] |
21.8
|
$ 0.0
|
| Operating Lease, Liability |
|
28.7
|
|
| Residual Value of Leased Asset |
|
$ 3.0
|
|
| Vehicles and Equipment [Member] |
|
|
|
| Lessee, Lease, Description [Line Items] |
|
|
|
| Operating Lease, Weighted Average Remaining Lease Term |
|
4 years
|
|
| Operating Lease, Weighted Average Discount Rate, Percent |
|
3.70%
|
|
| Land and Other [Member] |
|
|
|
| Lessee, Lease, Description [Line Items] |
|
|
|
| Operating Lease, Weighted Average Remaining Lease Term |
|
28 years
|
|
| Operating Lease, Weighted Average Discount Rate, Percent |
|
4.10%
|
|
|
|